Good tutorials for learning intermediate to advanced Regex?

So I'm familiar with the basics of regex but am looking for good tutorials for learning anything beyond the basics. I'd also appreciate any links to interactive problems where I can type in regular expressions and see if they solve it. Even ideas for intermediate or advanced problems I can try and solve through my own coding would be great.

From stackoverflow

• This is ground-zero for learning regular expressions:

  Regular Expression Tutorials

• Regular-Expressions info goes over every aspect of regular expressions.

  Also are some Advanced Regular Expression Examples that users have posted to test your knowledge

• Here you have a Regex Library. It's not exactly what you asked for, but you might find useful anyway: http://regexlib.com

• "Mastering Regular Expressions", by Jeffrey E. F. Friedl. Yes, it's a book vs. an on-line tutorial, but a very good one...

• I can really recommend www.regular-expressions.info along with RegexBuddy. Regexbuddy is a fantastic tool which helps you writing regex in various languages. It also has capabilities of explaining the regex for you.

  

• One program I always come back to is RegexDesigner. It's a very simple tool that let's you write, test, and save regexs; a lot like the above RegEx Buddy but simpler and free. It also generates C# and VB.NET code.

• How about re-builder in emacs